Oscillations of the large-scale circulation in turbulent mixed convection in a closed rectangular cavity

Fluid temperature time series are recorded in turbulent mixed convection at specific locations inside a cuboidal convection cell. They reveal instabilities of the large-scale flow structures, which organise the heat transport in an intermediate range of Archimedes numbers, where buoyancy and inertia forces are of similar strength. The instabilities lead to periodic or spontaneous transitions between three and four convection rolls. Further, for either high Rayleigh or Reynolds numbers, for which the flow is either governed by buoyancy or by inertia forces, respectively, stable large-scale circulations (LSCs) develop. In the intermediate Ra – Re number regime, we ascribe the complex dynamics, visible as oscillation in the temperature time series, to the interaction of the pressure-driven wall jet at the ceiling with the buoyancy-driven LSCs. The maximal main oscillation frequency is about one order of magnitude smaller than the turnover frequencies of either the wall jet-induced circulation rolls or thermally induced LSCs. It is further shown that the periodic reconfigurations of the LSCs can be controlled by adjusting the inflow velocity, that is, the Reynolds number, to generate stable LSCs.